1. Field of the Invention
The present invention relates to a device and method for attaching a threaded end connector to the end of a tube. More particularly, the invention relates to a device and method for attaching a threaded end connector to the end of a coiled tubing string so that it can connect to a variety of drillstring components.
2. Description of the Related Art
Coiled tubing offers a rapid and cost effective way to deploy and retrieve tools used in well drilling and servicing. Previously, the attachment of such tools or threaded adapters to the bottom end of a coiled tubing string has utilized either mechanical means or simple annular welds. Annular fillet or butt welds between the tubing and a tool or threaded adapter are difficult to reliably produce and can be subject to fatigue from bending. Allowable fillet weld sizes for connecting tubing to adapters are insufficient to develop the full axial and torsional strengths needed of the tubing.
One commercially available mechanical connection means, the External Dimple Connector, sold by Weatherford International Ltd., is engaged over the coiled tubing with a close slip fit and uses elastomeric seal rings to seal the annular gap. Multiple radially oriented grub screws, positioned in a regularly spaced pattern in the annular external wall of the Weatherford connector, move inwardly to plastically deform the tubing wall to create sufficient interlocking to provide the desired axial and torsional load capacity. However, this design can be ineffective if the elastomeric seals are damaged or deteriorate. An additional weakness of this type of connector can result if the grub screws are screwed in too little, resulting in an inadequate load capacity.
Weatherford International Ltd. also offers an Internal Dimple Connector. That connector also uses elastomeric seals to seal the annular gap between the tubing bore and the connector. The Internal Dimple Connector has multiple external semispherical pits into which a separate tool is used to plastically deform the tubing wall into each of the pits in response to radially inward thrusts applied by individual spherical ended screws. This type of connector also is sensitive to seal damage or deterioration. Because coiled tubing is typically formed with an internally protruding weld bead parallel to the tube axis, there is a likelihood of damage to the annular seals from contact with the weld bead, even if the tubing bore is reamed.
A third type of threaded adapter is the Coil Tubing Connector TT0740-375 from Thru-Tubing Technology. This connector also uses elastomeric seals to seal the annular gap between the tubing exterior and the connector bore. The connector consists primarily of two tubular bodies which are joined by a threaded connection. Axial and torsional gripping is provided by engagement of a deformable internally toothed, externally tapered annular slip retained in a frustroconical pocket of a first half of the connector. The second half of the connector abuts the lower end of the slip. Tightening the threaded connection between the first and second halves of the connector causes the slip to be axially compressed into the frustroconical pocket so that it is forced radially inwardly into engagement with the tubing. Radial set screws are used to prevent backing off of the threaded connection between the two halves of the connector. High friction in the threaded interconnection of the two connector halves can lead to inadequate axial capacity, and the elastomeric seals are subject to installation damage and deterioration.
A need exists for additional tubing connectors which can offer high axial and torsional strength along with reliable sealing between the connector and the tubing to which it is mounted.